Hyperthreading only duplicates the frontend of the CPU. That's really it. That's the entire explanation. It's useful if and only if there are unused resources

Hyperthreading only duplicates the frontend of the CPU. That's really it. That's the entire explanation. It's useful if and only if there are unused resources behind it, due to pipeline stalls or because the siblings are doing different things. It's virtually impossible to fully utilize a CPU core with a single thread; having two threads therefore boosts performance, but only to the degree that the first thread is incapable of using the whole thing. That's why the speedup is around 20%, not 100%.